The "bombe" was a significant development
in the evolution of the Allied cryptanalytic attack on the German Enigma
cipher machine. From 1928 to 1938, Polish cryptanalysts, aided by the French,
solved both the wiring of the three rotors used at the time and the indicator
system, and through the use of a small electrical machine using Enigma
rotors, created a card file which could anticipate the 6 by 17,576 possible
rotor positions used in setting up the Enigma. In 1938, however, the Germans
changed this entire indicator system.

To counter this move, the Poles created a hand
system with six sets of perforated sheets (each with 26 sheets) which recorded
about 1,000 possibilities on each sheet. When suitable combinations of
the sheets were superimposed, they enabled the Poles (and later Bletchley
Park) to determine the daily settings for Enigma. At the same time, they
devised a method of recovery consisting of six electrically powered sets
of Enigma wheels that rotated to search for repetitions in the "message
key" (the starting position of the rotors for a specific message)."

However, in December 1938 the Germans added
two more rotors to the three-wheel Enigma. Instead of six ways of arranging
the rotors, there were now sixty. Therefore, recovery required sixty perforated
sets (each containing 26 sheets) instead of six sets. Although the
Poles soon solved the wiring, they simply did not have the resources to
advance further on the bombe. At this point, the British assumed leadership
of the cryptanalytic attack on Enigma. Using all of the Polish data, and
that supplied by the French, the British decided on an entirely different
approach - a general method of recovering the wheel settings that did not
depend on solving the indicator system. Alan Turing and other British analysts
solved the problem, and the Turing prototype bombe appeared in May 1940.
The first British high-speed bombe was produced in April 1943. It was a
four-rotor bombe but Bletchley Park got no real value from it until June
[1].

In August of the same year, the Americans produced
their first bombe. Thereafter it was a matter of refining the technical
operation of the bombe and applying as many resources to the problem as
it took to maintain the production of intelligence. Production of the USN
bombes was pretty constant. Eighty bombes had arrived in Washington DC
by December 1943, 90 by June, and 115 by December 1944. The Germans made
very few changes to the operation of M-4 naval Enigma, which was
Op-20-G's primary target.

Early in WWII, President Roosevelt signed the
bill establishing the Women's Army Auxiliary Corps on May 15, 1942. A few
months later he signed a similar bill for the US Navy which created the
Women Accepted for Voluntary Emergency Service, more commonly known as
the WAVES. Only those women meeting higher qualifications were admitted
into cryptologic work and many operated the bombe.

The Navy also told their WAVES as little as
possible in order to maintain secrecy. They sent some 600 newly inducted
WAVES, along with 200 men, to Dayton, Ohio, to help build and train on
the cryptanalytic Bombes manufactured by the National Cash Register Company
. Each Bombe required sixty-four rotors to be wired to match the rotors
actually used in the Germans' Enigma. WAVES performed this task. Each woman
was given a wiring diagram for one side of the two-sided rotor. She spent
her eight-hour shift soldering wires to rotors. Another WAVE soldered the
other side. This arrangement maintained the secrecy of the rotor wiring.
No WAVE would have knowledge of both sides of any rotor.

Close-up views of a bombe rotor. The top photo
shows the rotor wiring. In the photo below it, details of the cover (left)
and commutator (right) can be seen. These components are on display at
the NCM.
(Photos by Jerry Proc)

Since the Allies did not know the German's
daily rotor selections, several Bombes worked on the same message. Each
Bombe tested a different set of wheel orders.The Bombes usually found two sets of possible
rotor settings on each run , but only one solution on one Bombe was the
correct wheel order and rotor position used by the Germans for that day.

After the Bombe completed a run, a WAVE supervisor
checked the printed results on the M-9 Bombe checker. She checked each
result looking for the correct one. Once she found the results ,
she used the M-9 to fill in any missing plugboard positions. The Bombes
could only find a portion of the Stecker positions because the menus were
between thirteen and sixteen letters long which was too short to find all
the plugboard connections.

Having found the correct wheel order, rotor
position and Steckers, the supervisor then sent the results back to the
library where WAVES and cryptananalysts used an analog and decrypted the
message. Short messages could be decrypted directly on the M-9 to work
against messages that had other problems such as garbles.

Eventually, the Navy built 121 Bombes
and sent them to the Naval Communications Annex. Here, women began operating
the machines. Three shifts of women ran them twenty-four hours a day. The
work was noisy and hot, but the Navy had impressed upon them the importance
of their work, without ever explaining how their work fit into the whole
cryptanalytic effort. Commander Gilman McDonnell, a supervisor on the Bombe
deck, recalled an incident in which a rotor solution, known as a jackpot,
was accidentally thrown out. The resulting delay made it apparent to the
chief of the Bombe operations, John Howard, that some kind of explanation
was necessary.

Eventually we got the answer and then they
realized that that wheel order had already been run and should've been
a jackpot. Well, it had obviously been thrown in the burnbag by mistake.
So John got permission from his superiors to tell the girls something to
give them some sense of how important the work was. We shut down operations
for about ten minutes and he got up on a chair in the middle of the operations
area. He didn't tell them specifically what they were doing, but said they
were attacking an enemy cipher and it was a very important job....and
that's all they knew.

Front view. The NCR Bombe as it is displayed
in the National Cryptologic Museum. (Photo by Jerry Proc)

The Op-20-G bombes were constructed at the U.S. Naval Computing Machine
Laboratory. Each bombe was the equivalent of 16 Enigma machines and consisted
of a set of 4 spindles and brush holders on which cross wired wheels were
loaded by hand.

MORE ON THE BOMBE

In February 2001, The Dayton Daily News published
an 8 part story featuring Joe Desch, the man who led the effort to manufacture
a practical, working machine. Select this link to read the story titled
"Dayton's Codebreakers".

The University of Dayton plans to demolish the building where the
bombe was developed. Efforts to save the buildings are told in this New
York
Times story dated April 1/2007.

FOOTNOTES:

[1] C. H. O'D. Alexander, Cryptographic History
of Work on the German Naval Enigma, p 53 (National Archives, Public Record
Office, Kew, Surrey, HW 25/1)